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Magnetism and the Motor Effect (EdExcel)
Magnets and Magnetic Fields
Magnets and Magnetic Materials:
Magnets attract magnetic materials. The main magnetic materials are:
Iron
Steel (an alloy of iron)
Nickel
Cobalt
Non-magnetic materials like wood, plastic, aluminium, and copper are not attracted to magnets.
Poles of a Magnet: Every magnet has two poles: a North-seeking pole (N) and a South-seeking pole (S).
Magnetic forces are strongest at the poles.
Like poles repel, and unlike poles attract. This is the fundamental rule of magnetism.
Magnetic forces are non-contact forces, meaning they can act at a distance.
Types of Magnets:
Permanent Magnets: These magnets produce their own persistent magnetic field. They are typically made of materials like steel and retain their magnetism over time. Examples include bar magnets, horseshoe magnets, and fridge magnets.
Induced Magnets: These materials become magnetic when they are placed in a magnetic field. They lose most or all of their magnetism when removed from the field. Soft iron is a good example of a material that easily becomes an induced magnet. Induced magnets always experience an attractive force towards a permanent magnet, never repulsion.
Magnetic Fields: A magnetic field is the region around a magnet (or a current-carrying wire) where a magnetic force is exerted on magnetic materials or other magnets.
Magnetic Field Lines: We represent magnetic fields using magnetic field lines (also called lines of force).
Field lines show the direction of the force that would act on a North pole placed at that point in the field.
Field lines always emerge from the North pole of a magnet and enter the South pole.
The closer the field lines are together, the stronger the magnetic field is in that region. The field is strongest at the poles where the lines are most concentrated.
Magnetic field lines never cross.
The direction of the magnetic field at any point is given by the tangent to the field line at that point.
Magnetic Field Patterns:
Bar Magnet: Field lines emerge from the North pole and curve around to enter the South pole. The field is strongest at the poles and weaker further away.
Between Two Like Poles (N-N or S-S): Field lines are repelled from each other, showing a weaker field in the region between the poles.
Between Two Unlike Poles (N-S): Field lines run directly from the North pole to the South pole, indicating a strong field in the region between the poles.
Earth's Magnetic Field: The Earth itself has a magnetic field, which is why a compass needle (a small magnet pivoted to rotate freely) aligns itself in a roughly North-South direction. This is thought to be due to the movement of molten iron in the Earth's outer core.
Plotting Magnetic Fields: You can plot magnetic field lines using:
Plotting Compasses: Place a small compass at different points around a magnet. The compass needle aligns with the magnetic field. Mark the direction of the needle and move the compass, continuing to mark directions. Connecting these marks creates field lines.
Iron Filings: Sprinkle iron filings around a magnet. The filings align themselves along the magnetic field lines, making the field pattern visible.
Electromagnetism
Magnetic Field around a Current-Carrying Wire: When an electric current flows through a wire, it creates a magnetic field around the wire.
The magnetic field is circular and is centred on the wire.
The field is strongest close to the wire and gets weaker as you move further away.
Direction of the Magnetic Field: You can determine the direction of the magnetic field using the Right-Hand Grip Rule (or Corkscrew Rule):
Point your right thumb in the direction of the conventional current (positive to negative).
Your fingers curl in the direction of the magnetic field (which is circular around the wire).
Reversing the direction of the current reverses the direction of the magnetic field.
Solenoids: A solenoid is a coil of insulated wire. It enhances the magnetic field effect of a single wire.
When a current flows through a solenoid, the magnetic fields around each loop of wire add together to create a stronger, more uniform magnetic field inside the solenoid and a field pattern outside resembling that of a bar magnet.
Magnetic Field Inside a Solenoid: The magnetic field inside a solenoid is strong and relatively uniform (field lines are parallel and equally spaced).
Magnetic Field Outside a Solenoid: The magnetic field outside a solenoid is similar to that of a bar magnet, with field lines emerging from one end (North pole) and entering the other end (South pole).
Poles of a Solenoid: A solenoid has a North and a South pole, just like a bar magnet. You can determine the poles using the Right-Hand Grip Rule again, but this time:
Curl your fingers in the direction of the conventional current around the coils.
Your thumb points towards the North pole of the solenoid.
Electromagnets: An electromagnet is created when a solenoid is wrapped around a ferromagnetic core (usually soft iron).
The iron core becomes magnetised by the magnetic field of the solenoid. The magnetic field of the core aligns with and greatly strengthens the magnetic field of the solenoid.
Electromagnets produce much stronger magnetic fields than solenoids alone.
Advantages of Electromagnets:
Can be turned on and off: The magnetic field exists only when current is flowing. Switching off the current switches off the electromagnet.
Strength can be varied: The strength of the electromagnet can be changed by:
Increasing the current flowing through the solenoid.
Increasing the number of turns (coils) in the solenoid.
Using a more magnetically permeable core material (though often soft iron is already optimal).
Applications of Electromagnets:
Relays: Use a small current in one circuit to control a larger current in another circuit. A low-voltage circuit energises an electromagnet, which attracts a switch to close a high-voltage circuit.
Electric Bells: When the circuit is closed, current flows through an electromagnet, attracting an iron armature. This causes a hammer to strike the bell. The movement also breaks the circuit, de-magnetising the electromagnet, and the armature springs back, re-closing the circuit, and the process repeats, ringing the bell.
Loudspeakers: An electromagnet is placed within the field of a permanent magnet. An alternating current in the electromagnet's coil causes the electromagnet's magnetic field to interact with the permanent magnet's field. This interaction causes the electromagnet (and the speaker cone attached to it) to move back and forth, creating sound waves.
Scrap Metal Cranes: Powerful electromagnets are used to lift and move large pieces of scrap iron and steel. The magnet can be switched off to drop the scrap metal.
The Motor Effect
The Motor Effect: When a current-carrying conductor (like a wire) is placed in a magnetic field, it experiences a force. This is known as the motor effect.
This effect is the basis for how electric motors work.
The force is perpendicular to both the direction of the current and the direction of the magnetic field.
If the wire is parallel to the magnetic field lines, it experiences no force. The force is maximum when the wire is perpendicular to the magnetic field lines.
Fleming's Left-Hand Rule: This rule helps you determine the direction of the force on a current-carrying conductor in a magnetic field.
Hold your left hand so that your thumb, first finger (forefinger), and second finger (middle finger) are at right angles to each other.
First Finger (Forefinger): Points in the direction of the Magnetic Field (from North to South).
Second Finger (Middle Finger): Points in the direction of the Conventional Current (from positive to negative).
Thumb: Points in the direction of the Motion (Force) on the conductor.
Remember: FBI - Force, Bield, Induction (Current). (Though 'Field' is more accurate than 'Flux' for GCSE level).
Factors Affecting the Strength of the Force: The strength of the force experienced by the conductor can be increased by:
Increasing the Magnetic Flux Density (B): This means using stronger magnets or concentrating the magnetic field. Magnetic flux density is measured in Tesla (T).
Increasing the Current (I): A larger current means more moving charges, and thus a stronger interaction with the magnetic field. Current is measured in Amperes (A).
Increasing the Length of the Conductor (l) in the Magnetic Field: A longer length of wire within the magnetic field will experience a greater force. Length is measured in metres (m).
Equation for Force: The force (F) on a current-carrying conductor in a magnetic field is given by the equation:
F = B × I × l
Where:
F = Force (in Newtons, N)
B = Magnetic flux density (in Tesla, T)
I = Current (in Amperes, A)
l = Length of the conductor in the magnetic field (in metres, m)
This equation is valid when the conductor is perpendicular to the magnetic field. If it's at an angle, the force is reduced.
DC Electric Motors: DC motors use the motor effect to convert electrical energy into kinetic energy (rotational motion).
Basic DC Motor Components:
Rotor (Armature): A coil of wire that is free to rotate.
Magnets: Permanent magnets (or electromagnets) to create a magnetic field.
Commutator: A split ring that reverses the direction of the current in the coil every half rotation.
Brushes: Conducting contacts that allow current to flow to the rotating commutator and coil.
How a DC Motor Works:
Current in the Coil: When a current flows through the coil in the magnetic field, forces act on the sides of the coil that are within the magnetic field (due to the motor effect).
Forces and Rotation: The forces on opposite sides of the coil are in opposite directions (use Fleming's Left-Hand Rule to verify). These forces create a turning effect (torque), causing the coil to start rotating.
Commutator Action: As the coil rotates past the vertical position, the commutator reverses the direction of the current in the coil. This is crucial because it reverses the direction of the forces on the coil sides, ensuring that the turning effect continues in the same direction, and the coil keeps rotating continuously in one direction.
Continuous Rotation: The repeated reversal of current by the commutator ensures continuous rotation of the coil as long as there is a current.
